Hydraulic oil well jar



June 20, 1961 J. DOWNEN HYDRAULIC on. WELL JAR Filed March 12, 1958 MIII-l1 2,989,132 HYDRAULIC OIL WELL JAR Jim L. Downen, 2931 Pierce Road,Bakersfield, Calif.,

assignor of one-half to Catherine A. Sutlifi, Bakersfield, Calif.

Filed Mar. 12, 1958, Ser. No. 720,912 5 Claims. (Cl. 175-297) Thisinvention relates to hydraulic well jars and has as one of its objectsthe provision of a hydraulic jar which is basically simple in design,relatively inexpensive to build, and yet is 'very rugged inconstruction, subject to few disorders, and therefore low in maintenancecosts.

The common practice in employing a jar in a drill string just above agrapple to assist in loosening a lost tool (termed a fish) forwithdrawing this from the well, is

to depend primarily on a series of upward jarring blows to loosen thefish. Each of these blows is eflected by a device in the jar whichautomatically temporarily restrains upward movement of the lower end ofthe drill string while the latter is stretched from its upper end, andthen suddenly relaxes said restraint to permit the tremendous energythus elastically stored in the drill string to lift a hammer shoulder inthe jar with a snap action over a short distance into impact with ananvil shoulder therein.

Drillers have found it useful in jarring operations to occasionallyalternate successive upward jarring blows with a downward blow againstthe fish. While jars have been provided to permit the driller the optionof effecting a snap action blow either upwardly or downwardly, the drillstring tends to buckle in the well when it is lowered to support asubstantial part of its Weight on the jar in order to perform a downwardsnap action blow through said jar. For delivering a downward blow in afishing operation, the drillers have therefore developed what is termedspudding the jar. The type of jar generally used in this is one providedwith means to utilize a tensioned drill string for striking a snapaction upward blow and which is then adapted to be telescopicallycollapsed relatively quickly to starting position. To facilitate thespudding operation, a bumper sub with a vertical lost motion of about 16inches is assembled between the drill string and the jar. There are twostyles of spudding operations, the more radical of which produces muchthe hardest downward blow but is likely to seriously damage the jar.This radical spudding operation immediately follows the upwardstretching of the drill stem to strike an upward snap action blowthrough the mechanism of the jar. As soon as this action is concludedthe driller suddenly drops the upper end of the drill string abouttwo-thirds of the total upward stretch required for the upward jar blow,and then resumes support of the drill stem. This snaps the lower end ofthe drill string downwardly with a rapid and powerful blow which istransmitted through the collapsed bumper sub to the upwardly extendedjar. This telescopically collapses the latter in a small fraction of asecond whereas it is designed for this collapsing movement to lastseveral seconds at least. While a terrific blow is thus transmittedthrough the jar to the fish, hydraulic jars subjected to this kind ofspudding action were damaged. In some instances the outer mandrel burstfrom the excessive internal oil pressure developed by the rapidtelescoping of the jar. In others the outer mandrel merely took anexpansive set.

This damage to the jars caused drillers to refrain from employing thisradical type of spudding action excepting when the damage suffered in itappeared justified in order to consummate a fishing operation withoutdelay. In ordinary practice, drillers have therefore substituted for themore radical spudding operation above-described one which is relativelycautious, and results in much less damage to the jars.

United States PatentO 2,989,132 Patented June 20, 1961 In preparing forthis somewhat milder mode of spudding, the driller first lowers thedrill string slowly so as to fully collapse both the jar and the bumpersub. He then lifts the drill string so as to extend the bumper sub toits full length while trying toleave the jar in its fully collapsedcondition. He now relaxes hold of the drill string where it is held atits upper end and resumes the support of it when it has fallen just lessthan the vertical distance of the free play in the bumper sub.

The kinetic energy thus set up in the drill string stretches this andforces the bumper sub downwardly rapidly a substantial distance afterthe sub has been completely collapsed. If the jar is collapsed when thisspudding action starts the downward blow thus delivered to the jar istransmitted thereby to the fish without any damage to the jar. It is notalways possible however to accurately determine the condition of the jarwhen this spudding action is started and sometimes the jar is at leastpartially extended when the blow is delivered to the jar and thussulfers damage by the building up of excessive internal oil pressureincidental to the telescoping of the jar by this blow.

The drillers of course are primarily concerned with successfullyterminating the fishing operation and are vexed by the necessity toexercise such extreme caution to prevent damaging the jar, especiallywhen it limits the intensity of the spudding blow which the equipmentcould otherwise deliver downwardly on the fish.

It is therefore another important object of the present invention toprovide a hydraulic oil well jar having means for utilizing a tensioneddrill string for striking a snap action upward jarring blow and which isadapted to sustain a radical spudding action at the time the jar isfully extended without suffering any damage therefrom.

In hydraulic jars generally of the prior art, the telescopic collapsingof the jar each time this is reset to start a snap action jarringoperation, requires a pumping of a certain volume of operating liquidthrough a relatively restricted passage and it is this feature whichcauses the jar to be damaged when it is spudded while the jar is infully extended condition.

Still another object of the invention is the provision of a hydraulicoil well jar which adjusts itself hydraulically to the telescopiccollapsing of the jar in resetting the latter for starting a new jarringoperation by the development of a vacuum in the high pressure chamber ofthe jar, thereby rendering the jar immune to damage by spuddingoperations even when the jar is in fully extended condition when thespudding blow is delivered thereto.

Another object of the present invention is the provision of a hydraulicoil well jar which includes means for breaking the aforesaid vacuum justprior to the completion of the telescopic movement for collapsing thejar thereby restoring internal operating liquid pressures in said jar tonormal.

Heretofore, hydraulic jars controllable by the relative movement betweenthe inner and outer tubular elements being retarded by a restrictedreverse flow of liquid past the piston as it travels through thecylinder, have made use of a loose fit between the piston and thecylinder to provide the escape passage. This has proven unsatisfactorybecause of the large change in rate of escape produced by a very smallchange in the ditference between the diameters of the piston and thecylinder.

The alternative of a small-diameter hole passing axially through thepiston has been tried. Such a hole, however, because of its smalldiameter and substantial length is not only diflicult to drill but tendsto clog up and thus impair the continued operation of the jar.

Yet another object of the invention is to provide a novel means forallowing a retarded escape of liquid past the piston in such a hydraulicjar at a precisely predetermined rate which will be free of all of theforegoing objections and will be relatively easy and inexpensive toproduce.

The manner of-aceomplishing the foregoing objects as well as furtherobjects and advantages will be made manifest in the followingdescription taken in connection with the accompanying drawings in whichFIG. 1 is a vertical sectional view of a hydraulic jar comprising apreferred embodiment of the present invention and illustrates the partsthereof disposed at a point during a resetting relative movement betweenthe outer and inner tubular elements of the jar when the piston isdisposed directly within the restricting cylinder of the jar, and showsa partial vacuum which has been drawn on the lower end portion of theliquid chamber on the line 3-3 of FIG. 1 and illustrates a liquid escapecheck valve of the invention which is provided in the annular floatingseal thereof.

FIG. 4 is an enlarged cross-sectional view, taken on the line 4-4 ofFIG. 1, and illustrating the manner in which a passage is formed in thejar of the present invention to permit a controlled by-passing of liquidpast the piston thereof while said piston is moving through saidcylinder.

FIG. 5 is an enlarged cross-sectional view taken on the line 5-5 of FIG.1 and illustrates the manner in which the outer and inner tubularelements of the jar of the invention are associated in splined relationwhereby rotary motion may be transmitted through said jar from the drillstring on the lower end of which said jar is suspended to a sub at thelower end of the jar which may be connected to the grapple or any otherpiece of oil well equipment with which the jar may be associated.

Referring specifically to the drawings the invention is shown therein asembodied in a hydraulic oil well jar which includes an outer tubularelement 11 and an inner tubular element 12 which are telescopicallyrelated to each other for relative axial movement in the operation ofthe jar. As shown in FIG. 1, the tubular element 11 forms a sleeve whichis internally threaded at its upper end so as to screw on an upper jarsub 13 which in turn is adapted to screw onto the lower pin 14 of atubular drill string 15 on which the jar 10 is suspended. The sub 13 hasa substantial cylindrical bore 16 the function of which will be madeclear hereinafter.

The inner tubular element 12 is externally threaded at its lower end andscrews into a lower jar sub 20 which has means for threadedly connectingit to the upper end of a grapple 21 designed for engaging an objectstuck in a well.

The tubular element 11 has a bore 22 at the lower end of which a counterbore 23 is provided, the lower end of said counter bore being threadedto receive a plug 24 for trapping a junk ring 25 and a mass of packing26 between said plug and said ring for forming a fixed seal means 27between the sleeve 11 and the inner tubular element 12 of the jar. Thebore 22 is also provided with spline grooves 28. The sleeve 11 likewisehas counter bores 29, 30 and 31, the counter bore 30 being somewhatlarger in diameter than the bore 22 and the counter bores 29 and 31having diameters greater than the counter bore 30 so that the lattercounter bore forms an annular radially inwardly projecting cylinder 32separating the counter bores 29 and 31. For purposes of distinguishingbetween counter bores 29 and 31, these may be referred to respectivelyas primary and secondary 4 counterbores. An annular radial impact face35 is provided on the sleeve 11 at the juncture of the bore 22 and thecounter bore 29.

The sleeve 11 is provided with a filler hole 36 and a filler plug 37 forclosing the same at a level just above the junk ring 25. This sleeve isalso provided with an air vent hole 38 and a plug 39 for closing thesame at a level just above the counter bore 30.

The internal tubular element 12 comprises a tubular mandrel of uniforminternal and external diameter throughout the length thereof exceptingfor the external threads at the lower end; for the provision of splines[formed thereon for interengagement with the spline grooves 28 of thesleeve 11, and excepting for the provision on said mandrel of an annularexternal shoulder 46 having a radial downward impact face 47, andexcepting for threads 48 provided on said mandrel just above theshoulder 46 onto which an internally threaded annulus 49 is adapted tobe screwed so that this forms a piston on said mandrel which is adaptedto move axially through the cylinder 32 with a close sliding fit, asshown in FIG. 1.

As is clearly shown in FIG. 1, the tubular elements 11 and 12 of the jar10 cooperate to provide, in the annular space between these elements, anoperating liquid chamber 50, one end of which is closed by the fixedannular seal 27 provided by the junk ring 25, packing 26 and plug 24.The other end of operating liquid chamber 50 is closed by a floatingseal 52 which comprises a metal ring 53 having annular grooves formedinternally and externally therein to accommodate O-rings 54 which form asliding liquid-tight sealing engagement between the ring 53 and theinner element 12 and O-rings 55 which form a liquid-tight slidingsealing engagement between the metal ring 53 and bore 31 of sleeve 11.The O-rings 54 and 55 thus operate to form a liquid tight seal betweenelements 11 and 12 within the bore 31 thereby forming an effectiveliquid-tight closure separating the liquid chamber 50 from the space inthe upper end of bore 31 which communicates through the bore 16 of thesub 13 with a mud passage of the drill string 15.

Confined in the chamber 50 of the jar 10 is a body of operating liquidwhich is ordinarily a light lubricating oil. This chamber is normallycompletely filled with this oil so that there is no air in chamber 50whatsoever. In some cases the ambient fluid surrounding the jar 10 willbe air but in most cases the ambient fluid both internal and external inwhich the jar 10 operates is rotary mud which flows downwardly throughthe mud passage 60 of the drill string, through the jar 10 and grapple21 and outward into the well (not shown) in which the jar 10 isoperating. Whatever this ambient fluid may be, however, it comes intocontact with the upper face of the floating seal 52 and also with thelower face of the fixed annular packing seal 27 and is completelyexcluded by these two seals from the liquid chamber 50.

Under abnormal operating conditions one or the other of these seals mayfail or wear so as to permit the admission of ambient fluid, eitherliquid or gaseous, into the liquid chamber 50. The present inventionenvisages this possibility and provides a means for ejecting from theliquid chamber 50 any excess quantity of fluid which might thus beadmitted to this chamber.

This means is comprised in a spring check valve 70 which is provided inthe metal ring 53, as illustrated in FIG. 3. This valve includes a bore71 which is formed vertically in the ring 53 and has a deep counter bore72 the lower end of which is internally threaded to receive a sleeve 73having a valve seat 74 formed on its inner end. Held on this seat by avery stiff coil spring 75 is a ball 76. The manner in which the valve 70functions to relieve the chamber 50 of excess fluid will be made clearin the description of the operation.

The peripheral face of the piston 49 is made to fit so closely with thecounter bore 30 of the cylinder 32 when said piston is passing throughthis cylinder than an inconsiderable amount of liquid is permitted toby-pass the piston. There is a definite reason for this close fitbetween the piston 49 and cylinder 32. It is to permit a closeregulation of the exact amount of liquid which is allowed to hy-pass thepiston 49 when a given pressure of liquid is imposed on this piston. Anaccurate determination of the rate of flow of liquid by-passing thepiston 49 is provided by forming a groove 86 vertically in the externalsurface 85 of the piston 49 so that practically the sole passage ofliquid past the piston 49 is the groove 86. While it is preferable toform the groove 86 in the peripheral surface 85 of the piston 49 thisgroove could of course, alternatively, be formed in the counter bore 30.For convenience however it is preferable to form groove 86 in the piston49 as above described.

Operation The preferred embodiment of the invention is shown in FIG. 2with the parts thereof properly related for starting a jarring operationwhich culminates in delivering a sharp upward blow to the grapple 21 andthe fish (not shown) engaged thereby.

Starting with the parts thus related, the jarring operation is effectedby applying an upward force on the jar by lifting on the drill stringfrom the surface of the well. As the string is lifted, the sleeve 11which is secured to the lower end of the drill string, is also liftedbringing the cylinder 32 into contact with the lower end of the piston49 and thus trapping operating liquid in the lower 1 of the sleeve 11 isretarded by the necessity of the operating liquid to escape from thelower end of chamber 50 upwardly past the piston 49. As above pointedout, the only escape provided for this liquid is through the groove 86,and this, of course, is relatively slow thereby permitting a tremendousupward strain or tension to be applied to element 11 through the drillstring 15 while the sleeve 11 is gradually pulled upwardly over thepiston 49 as the operating liquid escapes upwardly through the groove86.

When the sleeve 11 has thus been lifted to the point where the cylinder32 rises above the upper end of the peripheral surface 85 of piston 49,the liquid in the lower portion of the chamber 50 is free to flow fromsaid portion of chamber 50 upwardly around the piston 49 and thus freethe drill string from downward restraint allowing the drill string tosuddenly contract axially, imparting to the sleeve 11 a very rapidupward movement bringing the impact faces 47 and 35 into suddenengagement and striking a heavy blow upwardly on the annular shoulder 46of the element 12 of the jar. This blow of course is transmitted throughthis element and the sub to the grapple 21 which is engaged with thefish stuck in the well.

The normal resetting of the jar 10 to bring the parts thereof into therelative positions in which these are shown in FIG. 2, is accomplishedby lowering the drill string 15 and the sleeve 11 of the jar 10 untilthe plug 24 at the lower end thereof comes to rest on the sub 20.Assuming that the jarring operation is taking place near the bottom of aWell 7000 feet deep, for instance, the outside diameter of the jar asshown in the drawings would be 3". Thus, since the distance the sleeve11 travels downwardly from jarring relation with inner element 12 to itsreset relation therewith, as shown in FIG. 2 is about 1% times saidoutside diameter of the jar, the distance element 11 is thus lowered forresetting the jar is approximately 4 inches.

During the initial phase of this downward movement of -sleeve element11, the portion of oil chamber 50 embraced by counter bore 29 allows theoperating oil to flow freely axially past the piston 49, thereby settingup no resistance to such downward travel of element 11.

When the lower end of cylinder 32 reaches the upper end of piston 49,however, and starts to slide downwardly over this at a reasonable rateof down travel of the drill string 15 in this resetting operation,which, for instance, might be one inch per second, the close fit betweenthe piston 49 and cylinder 32 prevents a flow of oil between the pistonand cylinder excepting along groove 86, and, at this rate of descent ofsleeve 11, oil cannot flow downward through this groove fast enough tokeep the portion of chamber 50 embraced by counter bore 29' filled withoil.

A vacuum 90 is thus produced in the portion of chamber 50 just beneathcylinder 32. This vacuum imposes a resistance to the second half of thedownward movement of the sleeve element 11 in the resetting operation.This resistance is equal to the ambient fluid pressure multiplied by thenet area displaced by the piston 49. The ambient pressure at a depth of7000 feet being about 3010 p.s.i., and the net piston area about fourinches, the resistance offered by the vacuum 90 to the final 2% inchesof the downward travel of sleeve element Ji l is about six tons.

This is approximately the weight of the lowermost 420 feet of the drillstring which, in its entirety, is 7000 feet long and weighs over 100tons.

It is thus evident that the resistance offered by the vacuum 90 to thedownward movement of sleeve 11 has no substantial retarding effect onthis movement, the latter continuing at about the same rate it starteduntil it terminates with the plug 24 coming to rest on sub 20.

The resistance to this down movement is relaxed by the passage ofcylinder 32 downwardly from over the piston 49 just prior to the sleeve11 coming to rest on sub 20 so that there is no substantial downwardacceleration of sleeve element 11 during the very brief interval betweenthe dissipation of vacuum 90 and the end of said down movement of saidsleeve.

The resetting operation is therefore concluded, under the conditionsabove assumed, by the settling of the sleeve 11 of the jar 10 and aboutsix tons of drill string at a I speed of about one inch per secondontothe sub 20* of said jar.

The resetting operation is thus seen to be accomplished, in theembodiment of the invention disclosed herein, without delivering asubstantial downward blow to the fish grapple 21.

The weight of the drill string 15 is imposed on the grapple 21 ofcourse, as the resetting movement terminates, but the entire drillstring 15 is travelling at such a low speed when this weight transfertakes place as to have no practicalconsequence as a jarring action onthe grapple.

During downward movement of .the sleeve 11 in resetting the jar 10, asubstantial amount of the liquid in cham ber 50 remains above cylinder32 trapped between this cylinder and piston 49, on the one hand, and thefloating seal 52, on the other. This causes the floating seal 52 toremain spaced a substantial distance above the piston 49 as the sleeve11 moves downwardly. This almost brings the lower end of sub 13, whichconstitutes a stop for the upward movement of the floating seal 52, intocontact with this floating seal. Normally such contact does not takeplace, as the jar 10 is designed ot provide for free movement of thefloating seal 52 upwardly throughout the entire operation of resettingthe jar 10. In other words, as the cylinder 32 just starts to movedownwardly entirely away from around the piston 49, in this. resettingoperation, the floating seal ring 52 is still shy of engagement with thesub 13. At this precise point in the resetting operation, the upperportion of counter bore 29 cccupied by the vacuum is brought intocommunication with that portion of the liquid chamber 50 located abovesaid cylinder, whereupon operating liquid rushes downwardly between thecylinder 32 and piston 49 to fill said vacuum. This action of the vacuumof course sucks the floating seal ring 52 downwardly into much closerrelation with the piston 49 as this is shown in FIG. 2.

A complete cycle of the normal operation of hydraulic jar 10 has nowbeen described whereby the jar'has been returned to reset condition inreadiness for the starting of another upward jarring operation. It willbe appreciated that if, for any reason, a substantial amount of ambientfluid secures admission to the liquid chamber 50, this will causefloating seal 52 to engage the sub 13 during the next resettingoperation, before the latter is completed, whereupon the excess amountof fluid in chamber 50 will be expelled upwardly therefrom throughspring check valve 70.

In addition to its normal operation described hereinabove, the jar 10 ofthe present invention possesses the outstanding advantage of not settingup any liquid pressure pockets during its restoration to collapsedcondition as shown in FIG. 2, whereby the jar may be subjected to aradical spudding operation as heretofore described, without sufleringany damage whatsoever therefrom. In this spudding operation, a bumpersub (not shown) may or may not be assembled between the lower end of thedrill string 15 and the upper sub 13 of the jar 10. In either case theoperation is effected by performing an upward jarring action with thejar 10 terminating with this completely extended and with the drillstring 15 upwardly stretched quite a distance which is sometimes as muchas 6 feet.

While thus stretched and just after the upward jarring action in the jar"10 has taken place, the driller releases hold on the upper end of thedrill string 15 allowing the entire drill string to drop abouttwo-thirds the distance it had previously been stretched upwardly afterwhich the upper end of the drill string is caught and support thereofresumed allowing the drill string 15 to fall downwardly producing asnapping action at its lower end which impels the same downwardly at avery rapid rate thereby collapsing the jar 10 from its fully extendedposition to its fully telescoped position shown in FIG. 2 in a smallfraction of a second and striking a terrific spudding blow by the impactbetween the collar 24 and the sub 20. This blow of course is transmitteddownwardly to the fish with which grapple 21 is engaged.

The immunity of the jar 10 to damage by this very severe operation, isdue to the fact that its hydraulic adjustment during the telescopicreturn of the jar to collapsed condition is effected by developing avacuum 90 just below the cylinder 32 of the jar. This vacuum imposes aconstant resistance to the telescoping of the jar but it isinconsiderable in view of the tremendous pressures applied downwardlyagainst the jar by the radical spudding action above-described.Furthermore, the vacuum thus employed to adjust the resetting of the jardoes not impose any strains on the structure of the jar tending todamage this, and this is true irrespective of the speed with which thejar is telescoped.

It should be noted that there is a necessity for the release of thisvacuum by a free flow of oil into the space 90 at the termination of theresetting movement and this is accomplished by the relative shortness ofthe cylinder 32 and the provision for the piston 49 to be entirelybypassed by the downward movement of this cylinder as shown in FIG. 2thereby allowing a free flow of oil from the upper portion of thechamber 50 downwardly through the now open cylinder 32 into the lowerportion of chamber 50 where the vacuum 90 had existed.

This very useful advantage is coupled with the provision of a completelyfree floating seal ring 52 having ample room to slide vertically toadjust itself to changes in the amount of space occupied by liquidbetween the cylinder 32 and seal ring 52. Counter bore 29 may thus becharacterized as a high-pressure section of chamber 50 and the counterbore 31 may be characterized as a low pressure section of said chamber.The separation of these two counter bores by the relatively shortcounter bore 30 forming the cylinder 32 makes possible the performanceof all the functions above-described which take cally related'tubularelements; means for connecting one of said elements to a drill string;means for connecting the other element to an object to be jarred,telescopically overlapping portions of said elements providing anannular chamber for confining an operating liquid within said tool; anannular seal supported on the outer of said elements and slidablyengaging the other element to close one end of said chamber; an annularfloating seal disposed between and slidably engaging both of saidelements to close the other end of said chamber, both of said sealsbeingexternally in contact with ambient fluid; a relatively shortpiston, extending radially outwardly into said chamber from said innerelement; and a relatively short cylinder formed on said outer elementand extending radially inwardly therefrom into said chamber, said outerelement also being provided with primary and secondary counterbores,each having a substantially larger internal diameter than said cylinderand a substantially greater length than said piston, said primarycounterbore being located adjacent said cylinder between said cylinderand said fixed annular seal, said secondary counterbore being locatedaxially in the opposite direction from said cylinder and containing saidfloating seal, the latter moving freely axially in said secondarycounterbore in response to a differential between axial fluid pressuresthereagainst to confine said operating liquid in said chamber andexclude ambient fluid from said chamber, said piston and cylinderuniting when said piston is within said cylinder to comprise a liquidflow restricting means which limits the movement of operating liquidfrom one of said counterbores past said piston to the other of saidcounterbores to a flow at a relatively slow rate, relative axialmovement between said elements with said piston disposed within saidcylinder, in which movement the axial spacing of said piston from saidfixed seal increases at a relatively rapid rate, producing a vacuumwithin the primary counterbore portion of said chamber, while theoperating liquid in said secondary bore portion of said chamber occupiesthe space in said chamber between said piston and said floating seal andis maintained at ambient pressure by the free floating character of saidseal, said relative axial movement culminating in said cylinder movingslightly away from over said piston, in the direction therefrom of saidfixed seal, operating liquid thus being allowed to rapidly bypass saidpiston, flowing from said secondary counterbore through said cylinderand into said primary counterbore and relieving said vacuum, a reversemovement between said elements now moving said cylinder reversely oversaid piston at a positively retarded rate due to the necessity of theoperating liquid in the primary counterbore portion of said chamberescaping relatively slowly past said piston into said secondarycounterbore portion of said chamber, said elements being respectivelyprovided with axially opposed impact faces which are moved apartcoincidentally with the first aforesaid relative movement between saidelements producing a vacuum in said primary counterbore, and which aremoved together by said reverse relative movement of said elements andultimately at high velocity, for imparting to said object a jarring blowas said cylinder, in said reverse movement, passes away from over saidpiston.

2. A combination as in claim 1 in which limit stop means is provided forlimiting axial movement of said floating seal away from said cylinder;and spring check valve means provided in said floating seal to permitescape of liquid from said chamber into said ambient fluid when thefluid pressure within said chamber between said cylinder and saidfloating seal is caused to substantially exceed the ambient fluidpressure by engagement of said floating seal with said stop means,towards the conclusion of the first relative movement between saidelements as aforesaid.

3. A combination as in claim 1 in which said piston has a close,practically liquid-tight sliding fit with said cylinder, and in which agroove is formed in one of the annular meeting faces of said piston andcylinder, said groove connecting upper and lower edges of said face inwhich it is formed, whereby said groove provides a restricted passagefor slowly by-passing liquid past said piston during relative axialmovement between said piston and said cylinder while said piston is atleast partially disposed within said cylinder.

4. A combination as in claim 1 in which said piston has a close,practically liquid-tight sliding fit with said cylinder, and in which agroove is formed in the peripheral face of said piston, said grooveconnecting upper and lower edges of said face, whereby said grooveprovides a restricted passage for slowly by-passing liquid past saidpiston during relative axial movement between said piston and saidcylinder while said piston is at least partially disposed within saidcylinder.

5. In a hydraulic oil Well jar, the combination of: inner and outertelescopically related tubular elements; means for connecting one ofsaid elements to a drill string; means for connecting the other elementto an object to be jarred, telescopically overlapping portions of saidelements providing an annular chamber for confining an operating liquid;an annular seal supported on the outer element and slidably engaging theinner element to close one end of said chamber; an annular floating sealdisposed between and slidably engaging both of said elements to closethe other end of said chamber; a relatively short piston extendingradially outwardly from said inner element into said chamber; arelatively short cylinder formed on said outer element and extendingradially inwardly therefrom into said chamber into close slidingrelation 10 with said piston when the latter is within said cylinder,said cylinder dividing said chamber into a low pressure section,adjacent and containing said floating seal, and a high pressure section,adjacent said first mentioned seal, there being liquid passage meansallowing operating liquid to slowly by-pass said piston, flowing fromsaid high pressure chamber section to said low pressure chamber sectionwhen said piston is forced through said cylinder, by an extensivetelescopic motion between said elements, said motion between saidelements being relatively unrestrained immediately upon said motionbetween said piston and cylinder producing an axial gap between saidpiston and cylinder with said piston disposed in said high pressurechamber section; and impact shoulders provided on said elements inaxially opposed relation to he suddenly brought together to produce ajarring blow by said unrestrained extensive telescopic motion betweensaid elements, said low pressure chamber section being adequate inlength to permit a sudden compressive telescopic motion in the oppositedirection between said elements immediately following said blow, whichcauses said piston to quickly pass through said cylinder into said lowpressure chamber section, producing a temporary vacuum in said highpressure chamber section during such passage, and resetting said jar forthe initiation of another jarring action.

References Cited in the file of this patent UNITED STATES PATENTS1,637,505 Wigle Aug. 2, 1927 1,804,700 Maxwell May 12, 1931 2,265,431Kerr Dec. 9, 1941 2,645,459 Sutlifi July 14, 1953 2,802,703 HarrisonAug. 13, 1957

